The basic units of the Earth are rocks, and the basic units of rocks are minerals. The Earth itself is primarily composed of various minerals, of which over 4,700 have been identified so far. Common minerals in rocks include quartz, feldspar, calcite, biotite, muscovite, hornblende, pyroxene, and olivine. In daily life, the metals (such as gold, silver, copper, and iron) and non-metals (such as graphite, the main component of pencil cores, and sparkling gemstones on rings) we encounter are all processed from mineral raw materials.
Minerals are crystalline solid substances formed by geological processes, and their basic units are elements. Each mineral has a specific chemical composition, such as the chemical composition of calcite being calcium carbonate (CaCO3) and that of quartz being silica (SiO2). The atoms or ions that make up minerals arrange themselves in a regular and periodic manner in three-dimensional space, which forms the crystal structure of the mineral. Different ions of elements have different radii and chemical properties, and the arrangement of atoms or ions differs, resulting in different structures of minerals. In addition, geological conditions such as temperature and pressure also affect the arrangement of atoms or ions. Each mineral has its own specific structural form. If the composition is the same but the arrangement of atoms is different, it will exhibit different crystal structures and morphologies. For example, graphite and diamond, although both composed of carbon atoms, differ in their crystal structures due to the different arrangements of carbon atoms, resulting in huge differences in their morphologies and physical and chemical properties.
Usually, experienced people can identify the types of minerals based on their shapes, colors, luster, hardness, density, etc., such as pyrite, chalcopyrite, tourmaline, and diamond. There are thousands of minerals on Earth, but only 47 basic crystal forms, such as cubes and octahedrons. Why do minerals have regular shapes? This is because the growth process of minerals is a spontaneous accumulation of atoms and ions. The growth of each crystal face of a mineral crystal is controlled by its crystal structure. In a completely free growth space, the mineral will spontaneously form a regular geometric polyhedron according to the symmetry of its crystal structure. At this time, the symmetry of the external geometric shape of the crystal completely coincides with the symmetry of its crystal structure. When a mineral crystal does not have a free growth space, the growth of each crystal face will be limited by the space, resulting in an irregular shape. In the vast majority of cases, the shapes of minerals formed by geological processes deviate from the ideal shapes and present irregular shapes. Therefore, when observing the shapes of minerals, we should not only look at the size and shape of individual mineral particles, but also at the shapes of the mineral crystals arranged together.